Optical attenuator

ABSTRACT

A specific polarized light component out of a light wave passed through a magneto-optic crystal, for example, YIG is extracted by a polarizer. The intensity of the light beam output from the polarizer depends on strength and direction of magnetic fields applied to the magneto-optic crystal. The magneto-optic crystal is applied with a first and a second magnetic field acting in directions different from each other. The strength of the composite magnetic field of the first and the second magnetic fields is set to exceed a predetermined value at all times. By varying at least one of the first and second magnetic fields, the attenuation factor in the magneto-optic crystal can be changed continuously and with good reproducibility.

This application is a continuation of application Ser. No. 08/098,028,filed Jul. 28, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical attenuator including amagneto-optic crystal.

When conducting experiments with or adjustments of an opticalcommunication system, sometimes an optical attenuator is used foradjusting the intensity level of an optical input to any device which ispart of the system. As an optical attenuator, one adapted tomechanically vary its attenuation factor is known. However, in the casewhere an optical attenuator is incorporated in a system with theattenuation factor thereof being one of the objects to be controlled, itis desired that an optical attenuator having no mechanically moving partbe put to practical use to improve reliability on the system.

2. Description of the Related Art

As optical attenuators which have so far been in practical use, there isone in which the attenuation factor is changed by a mechanical motion.For example, having an attenuation film with a varying attenuationfactor distributed thereon inserted in the optical path, the attenuationfactor of the optical attenuator can be adjusted by shifting theattenuation film.

However, it sometimes becomes necessary in practice to use such anoptical attenuator incorporated in a control system in which theattenuation factor of the very optical attenuator is an object to becontrolled. An example is a case where, in an optical amplifier adaptedto amplify a signal light wave by conducting the signal light wavetogether with an pumping light wave through an optical fiber doped witha rare earth element such as Er (erbium), the power of the signal lightand/or pumping light is controlled in accordance with the monitoredlevel to thereby obtain a required characteristic (for example S/Nratio). In such a case, use of an optical attenuator adjusting theattenuation factor mechanically should be avoided in order to securereliability on the control system.

Accordingly, an object of the present invention is to provide an opticalattenuator having no mechanically moving part.

Other objects of the present invention will become apparent from thefollowing description.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an opticalattenuator varying an attenuation factor in its optical path inaccordance with an input signal thereto comprising a magneto-opticcrystal provided in the optical path, a polarizer provided in theoptical path on the downstream side of the magneto-optic crystalallowing a light wave having a specific direction of polarization topass therethrough, magnetic field application means for applying a firstand a second magnetic field acting in different directions from eachother to the magneto-optic crystal such that the strength of thecomposition of these magnetic fields exceeds a predetermined value, andmagnetic field adjustment means for varying strength of at least one ofthe first and second magnetic fields in accordance with the inputsignal.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram showing a basic structure of an opticalattenuator of the present invention;

FIG. 1B is a block diagram of an optical attenuator structured with amagneto-optic crystal and a polarizer in FIG. 1A;

FIG. 2 is a perspective view of an optical attenuator showing a firstembodiment of the present invention;

FIG. 3 is a diagram explanatory of magnetic field and magnetization inthe magneto-optic crystal shown in FIG. 2;

FIG. 4 is a perspective view showing another example of structure of aFaraday rotator shown in FIG. 2;

FIG. 5 is a diagram explanatory of magnetic field and magnetization inthe Faraday rotator shown in FIG. 4;

FIG. 6 is a perspective view showing a further example of structure ofthe Faraday rotator shown in FIG. 2;

FIG. 7 is a diagram explanatory of magnetic field and magnetization inthe Faraday rotator shown in FIG. 6;

FIG. 8 is a structural drawing of an optical attenuator showing a secondembodiment of the present invention;

FIG. 9 is a structural drawing of an optical attenuator showing a thirdembodiment of the present invention;

FIG. 10 is a perspective view showing an actually assembled state of theoptical attenuator shown in FIG. 9; and

FIG. 11 is a block diagram of an optical repeater to which the opticalattenuator according to the present invention is applicable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will be described belowin detail with reference to the accompanying drawings.

FIG. 1A is a block diagram showing a basic structure of an opticalattenuator of the present invention. Referring to FIG. 1A, referencenumerals 1 to 4 denote the above described magneto-optic crystal,polarizer, magnetic field application means, and magnetic fieldadjustment means, respectively. On the other hand, FIG. 1B is a blockdiagram showing a structure of an optical attenuator having nomechanically moving part using the magneto-optic crystal 1 and polarizer2 shown in FIG. 1A, the idea of which can be conceived of relativelyeasily. Referring to FIG. 1B, reference numeral 5 denotes a means forapplying a magnetic field to the magneto-optic crystal 1 in the samedirection as the propagating direction of light and reference numeral 6denotes a means for adjusting the strength of the magnetic field.Although it is unknown whether or not the optical attenuator shown inFIG. 1B has actually been proposed, the optical attenuator has beenshown as an example because it is considered useful for explaining theaction of the magnetic field application means 3 and magnetic fieldadjustment means 4.

Generally, under the condition of a magnetic field applied to amagneto-optic crystal, i.e., under the condition of a magneto-opticcrystal placed in a magnetic field, if linearly polarized light ispassed through the magneto-optic crystal, the direction of polarization(the projection of the plane containing the electric field vector of thelinearly polarized light on the plane perpendicular to the direction ofpropagation) is rotated in a fixed rotating direction at all timesregardless of the direction of propagation of light. This phenomena iscalled Faraday rotation. The magnitude of the angle of rotation of thedirection of polarization (the angle of Faraday rotation) depends on thedirection and strength of magnetization of the magneto-optic crystalresulting from the applied magnetic field. In concrete terms, the angleof Faraday rotation is determined by the magnitude of the component ofthe strength of the magnetization of the magneto-optic crystal in thedirection of propagation of the light wave. In an arrangement of amagneto-optic crystal and a polarizer in combination, if the angle ofFaraday rotation in the magneto-optic crystal is adjusted, the amplitudeof the light wave output from the polarizer can be changed in accordancewith the angle of Faraday rotation. Therefore, this arrangement isuseful in realizing an optical attenuator for linearly polarized light.

According to the arrangement of FIG. 1B, it seems, at first glance, thatthe angle of Faraday rotation in the magneto-optic crystal 1 can beeffectively adjusted by adjusting the strength of the applied magneticfield by the means 6. However, when the strength of the applied magneticfield is relatively small, the magnetization of the magneto-opticcrystal 1 by the applied magnetic field does not reach its saturatedstate. Hence a large number of magnetic domains within the magneto-opticcrystal 1 are present. Existence of such a large number of magneticdomains degrades the reproducibility of the attenuation factor in theoptical attenuator and, even if good reproducibility is secured,continuous variation in the attenuation factor becomes difficult toobtain. Further, when there exist a large number of magnetic domainswithin a magneto-optic crystal 1, attenuation due to scattering of lightby the boundary faces of these magnetic domains is produced and thisimpairs the practical use of the optical attenuator.

According to the present invention, since it is arranged such that themagnetic field application means 3 applies a first and a second magneticfield to the magneto-optic crystal 1 such that the strength of thecomposite magnetic field exceeds a predetermined value, keeping thestrength of magnetization in the magneto-optic crystal 1 saturated atall times is possible. Therefore, difficulties resulting from existenceof a large number of magnetic domains can be overcome. Here, the stateof a magneto-optic crystal in which the strength of magnetization issaturated can be considered to be a state where all the magnetic domainsare united. Further, according to the present invention, since it isarranged such that at least one of the first magnetic field and thesecond magnetic field is varied in strength by the magnetic fieldadjustment means 4. Changing the direction of the composite magneticfield of the first and second magnetic fields can be achieved. When thedirection of the composite magnetic field is changed, the direction ofthe magnetization in the magneto-optic crystal 1 is changedcorrespondently. As a result, although the strength of the magnetizationis saturated and kept constant, the component of the strength ofmagnetization in the direction of propagation of light changes and,consequently, the angle of Faraday rotation in the magneto-optic crystal1 changes. Thus, according to the present invention, an opticalattenuator having no mechanically moving parts and being better inreproducibility and more excellent in practicability than thearrangement of FIG. 1B can be provided.

FIG. 2 is a structural drawing of an optical attenuator according to afirst embodiment of the present invention. The optical attenuatorincludes a Faraday rotator 11 and a polarizer 12. The polarizer 12 isconstituted for example of a Glan-Thompson prism. The Faraday rotator 11is formed of a magneto-optic crystal 13, a permanent magnet 14 and anelectromagnet 15 applying the magneto-optic crystal 13 with magneticfields perpendicular to each other, and a variable current source 16applying a drive current to the electromagnet 15. By using such arelatively thin magneto-optic crystal 13 as to allow a light beam totransmit therethrough, it becomes possible to lower the saturationmagnetic field (the strength of the magnetic field required to saturatethe magnetization of a magneto-optic crystal, or to saturate the angleof Faraday rotation). As the magneto-optic crystal 13, sliced YIG(yttrium-iron garnet), epitaxially grown (GdBi)₃ (FeAlGa)₅ O₁₂, or thelike can be used. The direction of the magnetic field applied by thepermanent magnet 14 to the magneto-optic crystal 13 is parallel to thedirection of transmission of the light beam 17 through the magneto-opticcrystal 13, while the direction of the magnetic field applied by theelectromagnet 15 to the magneto-optic crystal 13 is perpendicular to thedirection of the magnetic field applied by the permanent magnet 14 andthe direction of the light beam 17 transmitted through the magneto-opticcrystal 13. The light beam 17 introduced into the magneto-optic crystal13 is linearly polarized light and the direction of polarization of thesame is caused to exhibit Faraday rotation by the Faraday rotator 11.Out of two polarization components whose directions of polarization areperpendicular to each other of the light beam passed through themagneto-optic crystal 13, one polarization component is extracted by thepolarizer 12 and this component becomes the optical output of theoptical attenuator. The strength of the composite magnetic field of themagnetic fields from the permanent magnet 14 and the electromagnet 15 isset to be greater than the saturation magnetic field in themagneto-optic crystal 13 at all times. The reason is as described above.The variable range of the variable current source 16 is set to a rangeby which the direction of polarization of the emitted light beam fromthe magneto-optic crystal 13 can be adjusted between the directionconcurrent with the direction of polarization of the optical output fromthe optical attenuator and the direction perpendicular to the same.

In the XYZ rectangular three-dimensional coordinate system used in thefollowing description, the Z-axis is parallel to the direction ofpropagation of the transmitted light through the magneto-optic crystal13 and the Y-axis is parallel to the direction of thickness of themagneto-optic crystal 13. Namely, in the present example, the directionof the applied magnetic field by the permanent magnet 14 is parallel tothe Z-axis and the direction of the applied magnetic field by theelectromagnet 15 is parallel to the X-axis.

FIG. 3 is a diagram explanatory of the directions and strengths of themagnetic field applied to the magneto-optic crystal 13 and themagnetization of the magneto-optic crystal 13. When a magnetic fielddenoted by reference numeral 101 is applied to the magneto-optic crystal13 only by the permanent magnet 14, the magnetization of themagneto-optic crystal 13 becomes parallel to the Z-axis as indicated byreference numeral 102. In this case, the strength of the appliedmagnetic field (the length of the magnetic field vector 101) is set to avalue by which the strength of magnetization of the magneto-opticcrystal 13 (the length of the magnetization vector 102) is saturated.Now, let it be assumed that an angle of Faraday rotation bringing thetransmittance of the optical attenuator to its maximum is beingobtained, for example, in this state. If a magnetic field from theelectromagnet 15 is applied parallel to the X-axis as indicated byreference numeral 103, the composite magnetic field becomes a compositevector of the magnetic field vectors 101 and 103 as indicated byreference numeral 104. By this composite magnetic field 104, there isproduced magnetization as indicated by reference numeral 105 in themagneto-optic crystal 13. The magnetization vector 105 and the magneticfield vector 104 are parallel to each other and the length of themagnetization vector 105 is in agreement with the length of themagnetization vector 102. Even if the strengths of magnetization of themagneto-optic crystal 13 are equal, it does not necessarily mean thatthe degrees of contribution made by them to the angle of Faradayrotation in the magneto-optic crystal 13 are equal. This is because theangle of Faraday rotation depends also on the relationship between thedirection of the magnetization and the direction of propagation oflight. More specifically, when the state where the magnetization 102 isexisting and the state where the magnetization 105 is existing arecompared, the Z component 106 of the magnetization 105 shows a decreaseagainst the Z component of the magnetization 102 (the magnetization 102itself), and the angle of Faraday rotation in the state of the latterbecomes smaller corresponding to that decrease. Since the maximumtransmittance is obtained in the state of the former, if it is desiredto obtain the minimum transmittance in the state of the latter, i.e., tohave all of the components of the light beam eliminated by the polarizer12 in the state of the latter, it will be achieved by setting thedifference in the angle of Faraday rotation between both of the statesto π/2 (90°). Expressing now the angle formed between the magnetic field101 from the permanent magnet 14 and the composite magnetic field 104 byθ, the angle formed between the magnetization 102 and the magnetization105 also becomes θ. Hence the ratio between the Z component 106 of themagnetization 105 and the magnetization 102 equals cos θ. If, forexample, the angle of Faraday rotation of the magneto-optic crystal 13is set to 2π when only the magnetic field from the permanent magnet 14is applied, the entire range from the maximum transmittance to theminimum transmittance can be covered by setting the range of variationof the current of the variable current source 16 such that (1-cos θ)becomes 0.25. When the corresponding angle of Faraday rotation is 4π and8π, the value of (1-cos θ) may become 0.25/2 and 0.25/4, respectively.

Therefore, if it is desired to hold down the range of variation of thecurrent to cover the entire range from the maximum transmittance to theminimum transmittance, it will be achieved, when the angle of Faradayrotation under the application of the magnetic field only from thepermanent magnet 14 is expressed by 2nπ (n is a natural number), bymaking n as great as possible. Although n was stated above to be anatural number for convenience of explanation, it may be some othervalue. The value of n can be set in accordance with the relationshipbetween the direction of polarization of the input light and thedirection of polarization of the transmitted light beam through thepolarizer 12. Since, according to the present embodiment, saturationmagnetic fields are applied to the magneto-optic crystal 13 throughoutthe range from where it provides the maximum transmittance to where itprovides the minimum transmittance, there arises no problem due toformation of a large number of magnetic domains within the magneto-opticcrystal 13. Further, since electrical adjustments are possiblethroughout the entire range from the point where the maximumtransmittance is given to the point where the minimum transmittance isgiven, it becomes possible to provide an optical attenuator having goodresponse and which is highly reliable.

FIG. 4 is a diagram showing a variation of the Faraday rotator 11 shownin FIG. 2. Points in which this Faraday rotator 11' are different fromthe Faraday rotator 11 in FIG. 2 are that planes 28 and 29 parallel toeach other are formed at opposing corner portions, i.e., at both endportions of a diagonal, of the magneto-optic crystal 13 and a light beam27 is arranged to penetrate these planes 28 and 29. In this example, thedirection of the magnetic field from the permanent magnet 14 and thedirection of the magnetic field from the electromagnet 15 are both at anangle of approximately 45° with the direction of propagation of thelight beam. In this example, in the XYZ rectangular coordinate system,it is set so that the Y-axis is parallel to the direction of thethickness of the magneto-optic crystal 13 and the Z-axis is parallel tothe direction of propagation of the light beam.

FIG. 5 is a diagram explanatory of the magnetic field and magnetizationin the Faraday rotator shown in FIG. 4. The magnetic field applied bythe electromagnet 15 is adjustable in strength and direction between thestate indicated by reference numeral 111 and the state indicated byreference numeral 112. Reference numeral 113 indicates the magneticfield applied by the permanent magnet 14. In this case, the compositemagnetic field varies in strength and direction over the range from thestate indicated by reference numeral 114 to the state indicated byreference numeral 115. In accordance with this, the magnetization of themagneto-optic crystal 13 varies in strength and direction over the rangefrom the state indicated by reference numeral 116 to the state indicatedby reference numeral 117. As apparent from the diagram, if the Faradayrotator 11' shown in FIG. 4 is used, the variable range of the abovedescribed angle θcan be easily set to a range from 0 to π/2. Themagnetic field applied by the permanent magnet 14 is set so that theangle of Faraday rotation will be sufficiently saturated in the statewhere the strength of the magnetization is at its minimum, as indicatedby reference numeral 118 (the state where the magnetic field applied bythe electromagnet 15 is zero).

FIG. 6 is a diagram showing another variation of the Faraday rotator ofFIG. 2. Points in which this Faraday rotator 11" are different from theFaraday rotator 11 of FIG. 2 are that an electromagnet 31 is used inplace of the permanent magnet 14 in FIG. 2 and further, a variablecurrent source 32 for supplying a drive current to the electromagnet 31is provided.

FIG. 7 is a diagram explanatory of the magnetic field and magnetizationin the Faraday rotator 11" of FIG. 6. According to the arrangement ofFIG. 6, the composite magnetic field can be varied continuously and withthe magnetic field kept in its saturated state as indicated by referencenumerals from 121 to 124. Accordingly, the magnetization of themagneto-optic crystal 13 is varied as indicated by reference numeralsfrom 125 to 128. Thus, according to the arrangement of FIG. 6, the rangeof variation of the above described angle θ can easily be set to thatbetween 0 and π/2 without using a magneto-optic crystal of a complicatedform such as shown in FIG. 4.

FIG. 8 is an structural drawing of an optical attenuator showing asecond embodiment of the present invention. There are arranged anoptical fiber 41, a lens 43, a wedge type double refraction crystal 44,the Faraday rotator 11 of FIG. 2, a wedge type double refraction crystal45, a lens 46, and an optical fiber 47 from the side of the lightsource, not shown, in the order named. The material of the doublerefraction crystals 44 and 45 is for example rutile (TiO₂) and they areof the same form. The top and the bottom of the double refractioncrystal 44 and the bottom and the top of the double refraction crystal45 are in opposing positions, respectively, and their correspondingplanes are parallel to each other. Further, the optical axes of thedouble refraction crystals 44 and 45 are on the plane perpendicular tothe paper. The relative position of the optical axes depends on thesetting of the loss at the time when the input to the variable currentsource of the Faraday rotator 11 is zero. In the following description,it is supposed that the setting is made such that the loss is reduced tothe minimum when the electrical input is zero and the optical axis ofthe double refraction crystal 44 and the optical axis of the doublerefraction crystal 45 are parallel to each other.

A light wave emitted from the excitation end of the optical fiber 41 iscollimated by the lens 43 to become a parallel light beam. This beam isindicated by reference numeral 130 with its thickness neglected. Thebeam 130 is separated in the double refraction crystal 44 into a beam131 corresponding to the ordinary ray and a beam 132 corresponding tothe extraordinary ray. The direction of polarization of the beam 131 andthe direction of polarization of the beam 132 are perpendicular to eachother. The beams 131 and 132 are caused by the Faraday rotator 11 torotate their directions of polarization the same angle and become beams133 and 134, respectively. The beam 133 is separated in the doublerefraction crystal 45 into a beam 135 as its ordinary ray component anda beam 136 as its extraordinary ray component. Also, the beam 134 isseparated in the double refraction crystal 45 into a beam 137 as itsextraordinary ray component and a beam 138 as its ordinary raycomponent. When the history of refraction each of the beams 135 to 138has undergone and the form and arrangement of the double refractioncrystals 44 and 45 are taken into consideration, the beams 135 and 137are parallel to each other and the beams 136 and 138 are not parallel toeach other. Accordingly, only the beams 135 and 137 out of the beams 135to 138 can be converged by the lens 46 to be introduced into the opticalfiber 47.

The ratio between the total power of the beams 135 and 137 to the totalpower of the beams 136 and 138 depends on the angle of Faraday rotationin the Faraday rotator 11. On the other hand, in the state where theangle of Faraday rotation in the Faraday rotator 11 is constant, thetotal power of the beams 135 and 137 does not depend on the state ofpolarization of the light wave emitted from the optical fiber 41.Therefore, according to the present embodiment, it is made possible toprovide an optical attenuator, of which the attenuation factor can bevaried continuously and electrically and the attenuation factor is notdependent on the state of polarization of the input light wave.

FIG. 9 is a structural drawing of an optical attenuator showing a thirdembodiment of the present invention. There are arranged an optical fiber51, a lens 52, a parallel-faced flat plate double refraction crystal 53,the Faraday rotator 11 of FIG. 2, a parallel-faced flat plate typedouble refraction crystal 54, a lens 55, and an optical fiber 56 fromthe side of the light source, not shown, in the order named. The doublerefraction crystals 53 and 54 are formed for example of rutile and areof equal thickness. The optical axis of the double refraction crystal 53and the optical axis of the double refraction crystal 54 areperpendicular to each other and each of the optical axes is at an angleof 45° with the direction of propagation of the light beam. A light waveemitted from the excitation end of the optical fiber 51 is collimated bythe lens 52 into a parallel beam. The beam is denoted by referencenumeral 140 with its thickness neglected. The beam 140 is separated inthe double refraction crystal 53 into a beam 141 corresponding to itsordinary ray and a beam 142 corresponding to its extraordinary ray. Thebeam 141 and the beam 142 are parallel to each other and the directionof polarization of the beam 141 and the direction of polarization of thebeam 142 are perpendicular to each other. The beams 141 and 142 arecaused by the Faraday rotator 11 to rotate their direction ofpolarization and become beams 143 and 144, respectively.

The beam 143 is separated in the double refraction crystal 54 into abeam 145 corresponding to its ordinary ray component and a beam 146corresponding to its extraordinary ray component. The beam 144 isseparated in the double refraction crystal 54 into a beam 147corresponding to its ordinary ray component and a beam 148 correspondingto its extraordinary ray component. Since the double refraction crystals53 and 54 are parallel to each other and are equal in thickness, thebeam 145 coincides with the beam 148. Accordingly, only the beams 145and 148 out of the beams 145 to 148 can be converged by the lens 55 andintroduced into the optical fiber 56.

The ratio between the total power of the beams 145 and 148 to the totalpower of the beams 146 and 147 depends on the angle of Faraday rotationin the Faraday rotator 11. On the other hand, in the state where theangle of Faraday rotation in the Faraday rotator 11 is constant, thetotal power of the beams 145 and 148 does not depend on the state ofpolarization of the light wave emitted from the optical fiber 51.Therefore, according to the present embodiment, it is made possible toprovide an optical attenuator of which the attenuation factor can bevaried continuously and electrically and the attenuation factor is notdependent on the state of polarization of the input light wave.

FIG. 10 is a perspective view of the optical attenuator of FIG. 9 in anactually assembled state. The end portion of the optical fiber 51 issupported by a ferrule 61 and the end portion of the optical fiber 56 issupported by a ferrule 62. In the description of FIG. 9, to make it easyto trace the light ray, a system of parallel beam is described to beformed between the lens 52 and the lens 55, but in actual assembly ofthe optical attenuator, it is arranged such that two focal points areformed between the lenses 52 and 55 as shown in FIG. 10. These focalpoints are located within the magneto-optic crystal 13 of the Faradayrotator 11. The light wave emitted from the excitation end of theoptical fiber 51 is converged by the lens 52 so that the beam diameteris reduced to the minimum within the magneto-optic crystal 13. The beamdiameter is enlarged again as it advances from the magneto-optic crystal13 to the lens 55. This beam is converged by the lens 55 and introducedinto the optical fiber 56. By structuring such an optical system, thebeam diameter within the magneto-optic crystal 13 can be made smallerand, hence, the use of the expensive magneto-optic crystal can bereduced to the minimum. In addition, the permanent magnet 14 andelectromagnet 15 for applying necessary magnetic fields to themagneto-optic crystal 13 can be made smaller.

FIG. 11 is a block diagram of an optical repeater to which the opticalattenuator of the present invention is applicable. An optical signalsupplied from an optical transmission line, not shown, is amplified byan optical amplifier 71. The amplified optical signal is passed throughthe optical attenuator 72 of the present invention and divided into twobranches in the optical branching circuit 73. One of the branched lightbeams is delivered to an optical transmission line, not shown, and theother of the branched light beams is converted to an electric signal inan O/E converter 74. This electric signal is supplied to a controller75. The controller 75 controls the attenuation factor of the opticalattenuator 72 such that the intensity of light received by the O/Econverter 74 becomes constant.

Since the optical repeater of the described type is frequently installedat such a place as the bottom of the sea, where maintenance of it isdifficult and each component of it is required to be highly reliable,the optical attenuator of the invention having no mechanically movingpart is most suitable for use as the component of such an opticalrepeater. Further, since the responding speed of the optical attenuatorof the invention is high, it can sufficiently respond to a quickvariation in the intensity level of the optical signal.

The present invention is not limited by the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. An optical attenuator comprising:a first opticalfiber having a first excitation end; a second optical fiber having asecond excitation end; a first lens and a second lens for opticallycoupling the first excitation end and the second excitation end by anoptical path; a first double refraction crystal and a second doublerefraction crystal provided on said optical path; and a Faraday rotatorprovided between the first and second double refraction crystals; saidoptical path being provided by a first beam and a second beamrespectively corresponding to an ordinary ray and an extraordinary rayin the first double refraction crystal, said first and second beamsbeing Faraday rotated by the Faraday rotator, said Faraday rotated firstbeam being split into a third beam and a fourth beam respectivelycorresponding to an ordinary ray and an extraordinary ray in the seconddouble refraction crystal, said Faraday rotated second beam being splitinto a fifth beam and a sixth beam respectively corresponding to theordinary ray and the extraordinary ray in the second double refractioncrystal, said third and sixth beams being input into the second opticalfiber from the second excitation end.
 2. An optical attenuator accordingto claim 1, wherein said Faraday rotator comprises:a magneto-opticcrystal supplied with said first and second beams; magnetic fieldapplication means for applying a magnetic field to the magneto-opticcrystal such that said first and second beams are Faraday rotated in themagneto-optic crystal, said magnetic field including a first magneticfield vector having a first direction and first strength and a secondmagnetic field vector having a second direction different from the firstdirection and a second strength, said first and second magnetic fieldvectors providing a resultant vector having a strength necessary tosaturate a strength of magnetization of the magneto-optic crystal; andcontrol means for continuously varying at least one of the first andsecond strengths in accordance with a control signal.
 3. An opticalattenuator according to claim 2, wherein said first and seconddirections are perpendicular to each other.
 4. An optical attenuatoraccording to claim 3, wherein said magnetic field application meansincludes an electromagnet and a permanent magnet for applying said firstand second magnetic field vectors, respectively, and said control meansvaries a current to drive the electromagnet.
 5. An optical attenuatoraccording to claim 4, wherein said second direction is substantiallyparallel to the optical path.
 6. An optical attenuator according toclaim 4, wherein each of said first and second directions inclinesapproximately 45° to the optical path.
 7. An optical attenuatoraccording to claim 3, wherein said magnetic field application meansincludes a first electromagnet and a second electromagnet for applyingsaid first and second magnetic field vectors, respectively, and saidcontrol means varies at least one current of a plurality of currents todrive said first and second electromagnets.
 8. An optical attenuatoraccording to claim 1, wherein each of said first and second doublerefraction crystals has a wedge form.
 9. An optical attenuator accordingto claim 1, wherein each of said first and second double refractioncrystals has a flat plate form.
 10. An optical attenuator according toclaim 1, wherein said Faraday rotator comprises:a magneto-optic crystalsupplied with said first and second beams; and magnetic fieldapplication means for applying a magnetic field to the magneto-opticcrystal such that said first and second beams are Faraday rotated in themagneto-optic crystal, said magnetic field including a first magneticfield vector having a first direction and a first strength and a secondmagnetic field vector having a second direction different from the firstdirection and a second strength, said first and second magnetic fieldvectors providing a resultant vector having a strength necessary tosaturate a strength of magnetization of the magneto-optic crystal.
 11. AFaraday rotator comprising:a magneto-optic crystal supplied with aninput beam; magnetic field application means for applying a magneticfield to the magneto-optic crystal such that the input beam is Faradayrotated in the magneto-optic crystal, said magnetic field including afirst magnetic field vector having a first direction and first strengthand a second magnetic field vector having a second direction differentfrom the first direction and a second strength, said first and secondmagnetic field vectors providing a resultant vector having a strengthnecessary to saturate a strength of magnetization of the magneto-opticcrystal; and control means for continuously varying at least one of thefirst and second strengths in accordance with a control signal, whereinsaid first and second directions are perpendicular to each other.
 12. AFaraday rotator according to claim 11, wherein said magnetic fieldapplication means includes an electromagnet and a permanent magnet forapplying said first and second magnetic field vectors, respectively, andsaid control means varies a current to drive the electromagnet.
 13. AFaraday rotator according to claim 12, wherein said second direction issubstantially parallel to the input beam.
 14. A Faraday rotatoraccording to claim 12, wherein each of said first and second directionsinclines approximately 45° to the input beam.
 15. A Faraday rotatoraccording to claim 11, wherein said magnetic field application meansincludes a first electromagnet and a second electromagnet for applyingsaid first and second magnetic field vectors, respectively, and saidcontrol means varies at least one current of a plurality of currents todrive said first and second electromagnets.
 16. An optical attenuatorfor attenuating an input light, comprising:a first double refractioncrystal separating the input light into a first light corresponding toan ordinary ray component and a second light corresponding to anextraordinary ray component; a Faraday rotator rotating a polarizationof the first light and a polarization of the second light; a seconddouble refracting crystal separating the polarization rotated firstlight into a third light corresponding to an ordinary ray component anda fourth light corresponding to an extraordinary ray component, andseparating the polarization rotated second light into a fifth lightcorresponding to an ordinary ray component and a sixth lightcorresponding to an extraordinary ray component; and an opticalwaveguide receiving the third and sixth lights.
 17. An opticalattenuator according to claim 16, wherein the Faraday rotatorcomprises:a magneto-optic crystal, the first and second lights passingthrough the magneto-optical crystal; magnetic field generating devicegenerating a first magnetic field having a corresponding strength and asecond magnetic field having a corresponding strength, the first andsecond magnetic fields combining together to form a resulting magneticfield applied to the magneto-optic crystal so that polarizations of thefirst and second lights are rotated as the first and second lights passthrough the magneto-optic crystal, the resulting magnetic field having adirection which is continuously variable through a specific range byvarying at least one of the strength the first magnetic field and thestrength of the second magnetic field; and a controller controlling atleast one of the strength of the first magnetic field and the strengthof the second magnetic field so that, as the direction of the resultingmagnetic field is varied through the specific range, the resultingmagnetic field maintains a strength which saturates a magnetization ofthe magneto-optic crystal.
 18. An optical attenuator according to claim17, wherein the first magnetic field is perpendicular to the secondmagnetic field.
 19. An optical attenuator according to claim 17, whereinthe magnetic field generating device comprises:an electromagnetgenerating the first magnetic field in accordance with a drive currentfor driving the electromagnet; and a permanent magnet generating thesecond magnetic field, the controller controlling the strength of thefirst magnetic field by varying the drive current of the electromagnet.20. An optical attenuator according to claim 19, wherein the first andsecond lights travel along an optical path, and the second magneticfield is substantially parallel to the optical path.
 21. An opticalattenuator according to claim 19, wherein the first and second lightstravel along an optical path, and the first and second magnetic fieldsare inclined approximately 45° to the optical path.
 22. An opticalattenuator according to claim 18, wherein the magnetic field generatingdevice comprises:a first electromagnet generating the first magneticfield in accordance with a drive current for driving the firstelectromagnet; and a second electromagnet generating the second magneticfield in accordance with a drive current for driving the secondelectromagnet, the controller controlling the strength of at least oneof the drive current of the first electromagnet and the strength of thedrive current of the second electromagnet.
 23. An optical attenuatoraccording to claim 16, wherein each of the first and second doublerefraction crystals has a wedge form.
 24. An optical attenuatoraccording to claim 16, wherein each of said first and second doublerefraction crystals has a flat plate form.
 25. A Faraday rotatorcomprising:a magneto-optic crystal having a light beam passingtherethrough; magnetic field generating device generating a firstmagnetic field having a corresponding strength and a second magneticfield having a corresponding strength, the first and second magneticfields combining together to form a resulting magnetic field applied tothe magneto-optic crystal so that a polarization of the light beam isrotated as the light beam passes through the magneto-optic crystal, theresulting magnetic field having a direction which is continuouslyvariable through a specific range by varying at least one of thestrength the first magnetic field and the strength of the secondmagnetic field; and a controller controlling at least one of thestrength of the first magnetic field and the strength of the secondmagnetic field so that, as the direction of the resulting magnetic fieldis varied through the specific range, the resulting magnetic fieldmaintains a strength which saturates a magnetization of themagneto-optic crystal, wherein the first magnetic field is perpendicularto the second magnetic field.
 26. A Faraday rotator according to claim25, wherein the magnetic field generating device comprises:anelectromagnet generating the first magnetic field in accordance with adrive current for driving the electromagnet; and a permanent magnetgenerating the second magnetic field, the controller controlling thestrength of the first magnetic field by varying the drive current of theelectromagnet.
 27. A Faraday rotator according to claim 26, wherein saidsecond direction is substantially parallel a travelling direction of thelight beam.
 28. A Faraday rotator according to claim 26, wherein thefirst and second magnetic fields are inclined approximately 45° to atravelling direction of the light beam.
 29. A Faraday rotator accordingto claim 25, wherein the magnetic field generating device comprises:afirst electromagnet generating the first magnetic field in accordancewith a drive current for driving the first electromagnet; and a secondelectromagnet generating the second magnetic field in accordance with adrive current for driving the second electromagnet, the controllercontrolling the strength of at least one of the drive current of thefirst electromagnet and the strength of the drive current of the secondelectromagnet.
 30. A Faraday rotator comprising:a magneto-optic crystalsupplied with an input beam; magnetic field application means forapplying a magnetic field to the magneto-optic crystal such that theinput beam is Faraday rotated in the magneto-optic crystal, saidmagnetic field including a first magnetic field vector having a firstdirection and a first strength and a second magnetic field vector havinga second direction different from the first direction and a secondstrength, said first and second magnetic field vectors combining to forma resultant vector having a direction which is continuously variablethrough a specific range by varying at least one of the first and secondstrengths; and control means for varying at least one of the first andsecond strengths so that, as the direction of the resultant vector isvaried through the specific range, the resulting vector maintains astrength which saturates a magnetization of the magneto-optic crystal,wherein said first and second directions are perpendicular to eachother.
 31. A Faraday rotator according to claim 30, wherein saidmagnetic field application means includes an electromagnet and apermanent magnet for applying said first and second magnetic fieldvectors, respectively, and said control means varies a current to drivethe electromagnet.
 32. A Faraday rotator according to claim 31, whereinsaid second direction is substantially parallel to the input beam.
 33. AFaraday rotator according to claim 31, wherein each of said first andsecond directions inclines approximately 45° to the input beam.
 34. AFaraday rotator according to claim 30, wherein said magnetic fieldapplication means includes a first electromagnet and a secondelectromagnet for applying said first and second magnetic field vectors,respectively, and said control means varies at least one current of aplurality of currents to drive said first and second electromagnets. 35.An apparatus comprising:a magneto-optic crystal having a light beampassing therethrough; magnetic field generating device generating afirst magnetic field having a corresponding strength and a secondmagnetic field having a corresponding strength, the first and secondmagnetic fields combining together to form a resulting magnetic fieldapplied to the magneto-optic crystal so that a polarization of the lightbeam is rotated as the light beam passes through the magneto-opticcrystal, the resulting magnetic field having a direction which iscontinuously variable through a specific range by varying at least oneof the strength the first magnetic field and the strength of the secondmagnetic field; and a controller controlling at least one of thestrength of the first magnetic field and the strength of the secondmagnetic field so that, as the direction of the resulting magnetic fieldis varied through the specific range, the resulting magnetic fieldmaintains a strength which saturates a magnetization of themagneto-optic crystal, wherein the first magnetic field is perpendicularto the second magnetic field.
 36. An apparatus according to claim 35,wherein the magnetic field generating device comprises:an electromagnetgenerating the first magnetic field in accordance with a drive currentfor driving the electromagnet; and a permanent magnet generating thesecond magnetic field, the controller controlling the strength of thefirst magnetic field by varying the drive current of the electromagnet.37. An apparatus according to claim 36, wherein the second magneticfield is substantially parallel to a travelling direction of the lightbeam.
 38. An apparatus according to claim 36, wherein the first andsecond magnetic fields are inclined approximately 45° to a travellingdirection of the light beam.
 39. An apparatus according to claim 35,wherein the magnetic field generating device comprises:a firstelectromagnet generating the first magnetic field in accordance with adrive current for driving the first electromagnet; and a secondelectromagnet generating the second magnetic field in accordance with adrive current for driving the second electromagnet, the controllercontrolling the strength of at least one of the drive current of thefirst electromagnet and the strength of the drive current of the secondelectromagnet.